Here is a pistol grip to fit any AR platform. I created this because I wanted a more upright grip for my AR pistol. I modeled the grip at a 12° angle, so your wrist won't need to bend so much.
Edit 7/16/2016: I added a rifle grip file that kicks the grip at a 20° angle which is more appropriate when the tool is being shouldered.
Align the base against the bed and only the inside pocket of the grip needs support. Make sure you use 100% infill; the walls are .1875" thick. I used taulman bridge filament and it is very solid feeling, but anything should work ok.